High-Speed Packet Access (HSPA) provides a major extension of the Wideband Code Division Multiple Access (WCDMA) radio interface. With HSPA it is now possible to provide mobile broadband since peak bit rates reach up to 42 Mbps in downlink using High-Speed Downlink Packet Access (HSDPA), and 11 Mbps in uplink using High-Speed Packet Uplink Access (HSUPA). The mentioned peak bit rates relate to standard specifications from 3rd Generation Partnership Project (3GPP) release 8. For 3GPP release 9 the peak bit rates are doubled. Thus, HSPA can be seen as a complement and replacement to other types of broad band access such as Asymmetric Digital Subscriber Line (ADSL).
To keep user equipment (UE) power consumption down most cellular systems has several states. In WCDMA there are five Radio Resource Control (RRC) states. These RRC states are Idle, CELL_DCH, CELL_FACH, URA_PCH, and CELL_PCH. Data transfer between the user equipment (also referred to as terminal herein) and the network is only possible in the CELL_FACH and CELL_DCH states.
From a user performance perspective it is desirable to keep downlink and uplink transport channels configured to rapidly be able to transmit any user data. At the same time, maintaining a connection in uplink (UL) and downlink (DL) comes at a cost. From a network perspective interference caused by control signalling that takes place even in absence of data transmission is a cost. From a UE perspective power consumption is a main concern.
In the CELL_DCH state dedicated channels in both the uplink and the downlink are used. The UE location is known with an accuracy of the cell. In a Serving Gateway Support Node (SGSN) the UE's serving Radio Network Controller (RNC) is known. The CELL_DCH state corresponds to continuous transmission and reception and allows for rapid transmission of large amounts of user data, but has the highest battery power consumption in the UE of the different RRC states.
The CELL_FACH state does not use dedicated channels and thus allows reduced battery power consumption, at the expense of a lower uplink and downlink throughput. The UE location is known with an accuracy of cell (in the SGSN with the accuracy of the serving RNC). In the CELL_FACH state the UE can transmit data as part of a random access procedure.
URA_PCH and CELL_PCH are states in which the battery power consumption is very low but still allow for reasonable fast transitions to the states in which data transfer can occur. The UE location is known with the accuracy of UTRAN Registration Area or cell respectively, however paging is needed to reach the UE. In URA_PCH and CELL_PCH the UE sleeps and only occasionally wakes up to check for paging messages.
The Idle state is the state that has the lowest battery power consumption, but the transition from the Idle state to a state in which data transfer can occur takes the longest time. The UE is known in the SGSN with an accuracy of routing area.
3GPP release 7 provided enhancements of the CELL_FACH state. The enhanced CELL_FACH has the possibility to utilize a High Speed Downlink Shared Channel (HS-DSCH) for downlink transmission instead of a Forward Access channel (FACH) which has a rather limited maximum bit rate. With the use of the HS-DSCH, the bit rates can be improved to around 300-500 Kbps compared to 32 Kbps when using FACH. Note however, that the RRC state is still called CELL_FACH even through the HS-DSCH is used. Sometimes the term enhanced CELL_FACH is used to specify that the HS-DSCH channel is used for downlink transmissions. In 3GPP release 8 a similar enhancement was made to the uplink when Enhanced Dedicated Channel (E-DCH) transmission in CELL_FACH was made possible for data transmission, the access procedure is still similar to that specified according to 3GPP Release 99. The above mentioned improvement in bit rate is considerable. However, for some real time applications, such as Push-to-talk over Cellular (PoC), another even more important improvement is an almost continuous transmission during channel switching between CELL_FACH and CELL_DCH in 3GPP Release 7 and later releases, while a channel switch between CELL_FACH and CELL_DCH may cause a transmission gap of several 100 ms in 3GPP Release 99. Compared to being in the CELL_DCH state and using HS-DSCH there is no Hybrid automatic repeat request (HARQ) procedure in the CELL_FACH state, but an initial link adaptation may be done using RRC measurement reports.
Enabling use of HS-DSCH in CELL_FACH (enhanced downlink CELL_FACH) will increase the throughput compared to CELL_FACH according to 3GPP Release 99 which used the FACH. Correspondingly the throughput in the uplink will increase where the E-DCH is used to transmit the data which according to 3GPP release 99 would have been transmitted using a Random Access Channel (RACH) to transmit the data. There is a reduced need for processing and memory hardware in a radio base station (referred to as NodeB according to 3GPP terminology) when the UE is in CELL_FACH compared to when it is in the CELL_DCH state. The improved bitrates offered by HS-DSCH and/or E-DCH combined with the reduced need for processing and hardware and reduced channel switching signalling overhead means that it is highly beneficial to keep the UEs in the CELL_FACH state when transmitting intermittent bursts of data, instead of switching up to the CELL_DCH state.
However, there is only a limited link adaptation, based on RRC measurement reports, which can be used in enhanced DL CELL_FACH. This means that the transmission margins on power, supported transport format (bit rate) and number of fixed HARQ transmission need to be quite high. The larger margins are a waste of resources and lead to a lower performance and utilization of the enhanced DL CELL_FACH. So it is not efficient to transmit data continuously using enhanced DL CELL_FACH. In this case the user should be switched up to the CELL_DCH state.
There are current state of the art state switching mechanisms that use an algorithm that takes into account the amount of data a UE has to transmit along with a Radio Link Control (RLC) buffer threshold level. This will lead to up-switch of users who have a single large packet to transmit while at the same leaving users with continuous flow of small sized packets in CELL_FACH. This is generally not a preferred behaviour.
In typical implementations, the RLC buffer threshold is fixed to a given value that is the same for all users, or set differently per group or per UE, depending on user data characterization. However, selecting the RLC buffer threshold based on the user data characterization has the drawback that it requires deep packet inspection (DPI). DPI is done in the core network and not in the radio access network (RAN). Another drawback is that it requires keeping a lot of statistics per UE in the core network and ways to communicate to different nodes that take part in the state switching decisions. These drawbacks require heavy processing power, memory requirements, and standardized methods or protocols to communicate the information to the different nodes. Standardized methods or protocols are needed if the state switching algorithm is to work within nodes from different companies. Note also that DPI is problematic when the user runs a number of applications simultaneously, which is common on today's mobile broadband connections.
Hence, there is a need for a procedure that overcomes at least some of the drawbacks above.